1. Technical Field
This invention relates to a microfluidic device that has micro-sized flow paths dug into a glass substrate or a plastic substrate so as to make an analysis or produce a reaction in the flow paths by use of small amounts of samples and, more specifically, to a micropump that generates a flow in the direction of a flow-path axis while driving liquids in flow paths and to a micromixer that stirs and mixes liquids together while generating a swirling flow. Additionally, this invention relates to an analytical instrument that uses liquid or a particulate material flowing in liquids as a sample and that measures progress information about reactions to a reagent or collects reaction products.
2. Description of the Related Art
In recent years, to reduce the amount of samples and save process steps, a reactor or an analysis method that uses a microfluidic device widely has spread. An electro-osmotic flow or a pressure flow is widely used as a method for conveying liquids in the microfluidic device. However, disadvantageously, many high-voltage power supplies and many pumps will be needed, and peripheral devices will be made large in size if micro-sized flow paths are complexly structured. Additionally, there remain unsolved problems, such as the “dead volume” problem of being incapable of reducing the rate of useless samples by use of an electro-osmotic flow or a pressure flow although the amount of samples to be used has become small by micronizing a flow path.
To make the whole device compact, a pump formed in the microfluidic device has been designed. Examples of such pumps include a mechanical pump using a diaphragm shown in Patent Document 1 and an electric pump using the action of an AC electro-osmotic flow shown in Non-Patent Document 1. However, the mechanical pump has defects one of which is the fact that special materials, such as piezoelectric material and bimetal, are needed and another one of which is the fact that many production processes must be followed. Therefore, a rise in manufacturing costs is caused, and a complex structure having great “dead volume” is formed. Additionally, disadvantageously, clogging is liable to occur, and a pulsating flow is caused. In contrast, the electric pump advantageously has a simple structure. However, the electric pump is not operated with the electrical conductivity (1.6 siemens per meter (S/m)) of a physiological saline used and important in medical and biological fields, and is only operated with the electrical conductivity of a liquid which is equal to or less than 1/100 (i.e., about 10 millisiemens per meter (mS/m)) of that of the physiological saline at a maximum.
On the other hand, the microfluidic device is characterized in that a diffusion-controlled chemical reaction is accelerated by a size effect, in that a slight amount of fluid is treated in a tightly-sealed state, hence in that environmental pollution can be prevented, in that a temperature-control response is swift, in that a reaction field having no temperature distribution can be obtained, and in that poisonous materials or an unstable, explosive sample can be managed under safe environmental conditions. Therefore, the microfluidic device also has been highly expected as a microchemical reactor. However, disadvantageously, it is difficult to secure a necessary reaction time, because one of the restrictions imposed on the microfluidic device is that the flow path, which is a reaction field, is short.
To hasten the reaction time, various mixers have been designed. Examples of such mixers include a hydrodynamic mixer (chaotic mixing) in which an obstacle is placed in a flow path as shown in Patent Document 2 and an electric mixer that uses an electrothermal effect or an AC electro-osmotic flow as shown in Non-Patent Document 2 and Patent Document 3. However, the hydrodynamic mixer needs a special microfabrication technique, and has many production processes, and hence is high in manufacturing costs. In addition, disadvantageously, the hydrodynamic mixer has a complex flow path structure that easily causes clogging, and has great flow path resistance resulting from the use of a flow force for stirring. On the other hand, the electric mixer has the advantage of having a simple structure as already described in the pump. However, the electric mixer is not operated with the electrical conductivity of a physiological saline used and important in medical and biological fields, and is only operated with the electrical conductivity of a liquid which is equal to or less than 1/100 of that of the physiological saline at a maximum.    [Patent Document 1] WO 98/51929    [Patent Document 2] WO 03/011443    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2006-320877    [Non-Patent Document 1] A. B. D. Brown, C. G. Smith, and A. R. Rennie: “Pumping of water with ac electric fields applied to asymmetric pairs of microelectrodes”, Physical Review E, vol. 63, 016305 (2000)    [Non-Patent Document 2] Marin Sigurdson, Dazhi Wang, and Carl D. Meinhart: “Electrothermal stirring for heterogeneous immunoassays”, Lab on a Chip, vol. 5, pp. 1366-1373 (2005)